1. Field of the Invention
The present invention relates to an adaptive equalizer device and a method for controlling an adaptive equalizer which are applied to technical fields such as digital communication for digital cable television (CATV), for example.
2. Description of the Background Art
Digital communications such as digital cable television are coming into practice as high-speed data communication systems replacing conventional analog communications. In a digital communication like the digital cable television, a received signal IN transmitted from a transmitting side to a receiving side has the features shown in FIG. 12. That is to say, in a QAM (Quadrature Amplitude Modulation) transmission/reception method, the received signal IN is transmitted for each predetermined period (symbol period SP), as n, n+1, n+2, . . . . The received signal IN includes transmitted data called symbols and a carrier at a high frequency for carrying the symbols.
The digital cable television is susceptible to multipath transmission in which radio waves propagate through a plurality of paths because of reflection at an end of a cable, for example. The presence of multiple delay waves has been confirmed, which occur when a plurality of radio waves caused by multipath transmission overlap. The multiple delay waves cause frequency-selective fading, and the frequency-selective fading causes distortion in the received signal IN.
Next, a conventional receiver will be described. FIG. 13 is a block diagram showing part of a structure of a conventional receiving processing circuit of QAM transmission/reception system. First, an I/Q demodulator 1 removes carrier from the received signal IN and separates the received signal IN into I (In-phase) component xi and Q (Quadrature) component xq. The input signal x(n) indicates the component xi or the component xq. The character (n) indicates that it is a signal in the nth symbol period SP.
The carrier is not completely removed by the I/Q demodulator 1, but may remain in the input signal x(n). The carrier remaining unremoved in the input signal x(n) is referred to as a carrier frequency error.
Next, the carrier frequency error, distortion due to frequency-selective fading, and other components in unwanted frequency bands and noise are removed from the input signal x(n), to obtain an output signal y(n). That is to say, the matched filter 2 removes components in unwanted frequency bands and noise from the input signal x(n). The carrier frequency error removing circuit 3 removes the carrier frequency error from the input signal x(n). The adaptive equalizer 4 removes the distortion due to frequency-selective fading from the input signal x(n), and outputs the distortion-removed input signal x(n) as the output signal y(n). The carrier frequency error detecting circuit 5 detects the carrier frequency error from the output signal y(n) and feedback controls the carrier frequency error removing circuit 3 with the detected result R1. Thus the carrier frequency error removing circuit 3 more completely removes the carrier frequency error from the input signal x(n).
FIG. 14 is a conceptual diagram showing an example of the output signal y(n). The output signal y(n) is composed of 64 pieces of data, for example. The 64 pieces of data each include I component yi and Q component yq. In FIG. 14, the components yi and yq are plotted as dots on the I-Q coordinates, which is called a symbol point arrangement diagram. The character S in FIG. 14 indicates a symbol, which is a group of 64 dots. In FIG. 14, 64 dots form a symbol S, which is called 64-value QAM. When an carrier frequency error occurs, the symbol S rotates as shown in FIG. 15, for example. When the symbol S rotates, processing to the received signal IN becomes difficult.
The carrier frequency error causes the symbol S to rotate for the following reason. In the QAM transmission/reception method, as shown in equation (1) below, the I axis component zi is multiplied by a cosine signal cos(wt) and the Q-axis component zq is multiplied by a sine signal sin(wt), and a transmitted signal IN0 obtained by adding them is sent from the transmitting side to the receiving side.
IN0=zixc2x7cos(wt)+zqxc2x7sin(wt)xe2x80x83xe2x80x83(1)
In equation (1), w indicates the carrier frequency of the transmitted signal IN0, and t indicates time. The received signal IN is expressed by equation (2) below.
IN=zixc2x7cos(wat)+zqxc2x7sin(wat)xe2x80x83xe2x80x83(2)
In equation (2), wa indicates the carrier frequency of the received signal IN.
On the receiving side, the input signal N is multiplied by a cosine signal cos(wt), as shown in equation (3).
INxc2x7cos(wt)=zixc2x7cos(wt)xc2x7cos(wat)+zqxc2x7sin(wt)xc2x7cos(wat)=zixc2x7[cos{(wxe2x88x92wa)t}+cos{(w+wa)t}]/2+zqxc2x7[sin{(w+wa)t}+sin{(wxe2x88x92wa)t}]/2xe2x80x83xe2x80x83(3)
Only the low-frequency component is taken out from equation (3), as xi. That is to say,
xi=zi/2xc2x7cos{(wxe2x88x92wa)t}+zq/2xc2x7sin{(wxe2x88x92wa)t}xe2x80x83xe2x80x83(4)
Further, on the receiving side, the input signal IN is multiplied by a sine signal sin(wt), as shown in equation (5).
INxc2x7sin(wt)=zixc2x7cos(wt)xc2x7sin(wat)+zqxc2x7sin(wt)xc2x7sin(wat)=zixc2x7[sin{(w+wa)t}xe2x88x92sin{(wxe2x88x92wa)t}]/2+zqxc2x7[cos{(wxe2x88x92wa)t}xe2x88x92cos{(w+wa)t}]/2xe2x80x83xe2x80x83(5)
Only the low-frequency component is taken out from equation (5), as xq. That it to say,
xq=xe2x88x92zi/2xc2x7sin{(wxe2x88x92wa)t}+zq/2xc2x7cos{(wxe2x88x92wa)t }xe2x80x83xe2x80x83(6)
The I/Q demodulator 1 on the receiving side takes out the components xi and xq from the received signal IN in this way.
In equations (4) and (6), when wa is equal to w, then
xi=zi/2
xe2x80x83xq=zq/2
Where no frequency component remains. That it to say, no carrier frequency error occurs.
However, when wa is not equal to w, frequency component remains. That is to say, a carrier frequency error occurs. Equations (4) and (6) can be expressed in a matrix as                               (                                                    xi                                                                    xq                                              )                =                              (                                                                                cos                    ⁢                                          {                                                                        (                                                      w                            -                            wa                                                    )                                                ⁢                        t                                            }                                                                                                            sin                    ⁢                                          {                                                                        (                                                      w                            -                            wa                                                    )                                                ⁢                        t                                            }                                                                                                                                                              -                      sin                                        ⁢                                          {                                                                        (                                                      w                            -                            wa                                                    )                                                ⁢                        t                                            }                                                                                                            cos                    ⁢                                          {                                                                        (                                                      w                            -                            wa                                                    )                                                ⁢                        t                                            }                                                                                            )                    ·                      (                                                                                zi                    /                    2                                                                                                                    zq                    /                    2                                                                        )                                              (        7        )            
Equation (7) shows rotation. It is seen from equation (7) that the presence of carrier frequency error causes the symbol S to rotate.
FIG. 16 shows basic structure of the conventional adaptive equalizer 4. The conventional adaptive equalizer 4 includes a discrete filter 6, which serves as the main element, an error detecting circuit 7, and a coefficient updating circuit 8. The discrete filter 6 includes shift registers SR0 to SRLxe2x88x921, multipliers M0 to MLxe2x88x921, and adders A1 to ALxe2x88x921.
Next, operation made by the discrete filter 6 will be described. The discrete filter 6 removes distortion caused by frequency-selective fading from the input signal x(n) and outputs the distortion-removed input signal x(n) as the output signal y(n). The shift registers SR0 to SRLxe2x88x921 delay the input signal x(n) by an amount of delay, Zxe2x88x921. Next, the multipliers M0 to MLxe2x88x921 multiply signals at the respective output nodes (referred to as xe2x80x9ctapsxe2x80x9d) of the shift registers SR0 to SRLxe2x88x921 and the coefficients C0 to CLxe2x88x921. Next, the adders A1 to ALxe2x88x921 add the multiplied results obtained by the multipliers M0 to MLxe2x88x921. The sum of the multiplied results from the multipliers M0 to MLxe2x88x921 corresponds to the output signal y(n).
The distortion due to frequency-selective fading may not be completely removed by the discrete filter 6, but may remain in the output signal y(n). The distortion due to frequency-selective fading remaining unremoved in the output signal y(n) is referred to as a distortion error. When the distortion error is large, it causes intersymbol interference. The intersymbol interference means interference in which symbols S transmitted in respective symbol periods SP interfere with each other. On an ideal transmission path with no multipath transmission, no distortion is caused by frequency-selective fading and therefore a certain symbol S does not affect other symbols S transmitted in other symbol periods SP. That is to say, no intersymbol interference occurs. On the other hand, when multipath transmission causes multiple delay waves to occur, frequency-selective fading causes distortion, and then the delay will cause a plurality of symbols S to arrive at the receiving side in the same symbol period SP. That is to say, intersymbol interference occurs.
Accordingly, the error detecting circuit 7 shown in FIG. 16 detects distortion error e(n) from the output signal y(n) to prevent the intersymbol interference. Next, the coefficient updating circuit 8 calculates and updates the coefficients C0 to CLxe2x88x921 on the basis of the distortion error e(n) and the input signal x(n). That is to say, the error detecting circuit 7 and the coefficient updating circuit 8 feedback controls the discrete filter 6. Thus the output signal y(n) becomes closer to an ideal output signal y(n), or an output signal y(n) from which distortion due to frequency-selective fading has completely been removed.
Operations made by the error detecting circuit 7 and the coefficient updating circuit 8 are disclosed in Jablon N. K., xe2x80x9cJoint Blind Equalization, Carrier Recovery, and Timing Recovery for High-Order QAM Signal Constellations,xe2x80x9d IEEE Transactions on Signal Processing, Vol. 40, No. 6, pp 1383-1398, June 1992 (cited reference 1), for example. According to the cited reference 1, first, the coefficients C0 to CLxe2x88x921 at the taps calculated by the coefficient updating circuit 8 are obtained as
Ch(n+1)=Ch(n)xe2x88x92uxc2x7e(n)xc2x7x(nxe2x88x92h)xe2x80x83xe2x80x83(8)
In equation (8), h is one of 0 to Lxe2x88x921. The character u is a constant which ensures convergence of the output signal y(n). The coefficient updating method expressed by equation (8) is called least mean square error (LMS) method.
The algorithm with which the carrier frequency error detecting circuit 5 in FIG. 13 detects the carrier frequency error is disclosed in Yamanaka K., et al., xe2x80x9cA multilevel QAM Demodulator VLSI with Wideband Carrier Recovery and Dual Equalizing Mode,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol. 32, No. 7, pp. 1101-1107, July 1997 (cited reference 2).
According to the cited reference 2, for example, the carrier frequency error detecting circuit 5 is constructed as shown in FIG. 17. First, the power calculating circuit 9 calculates electric power of the output signal y(n), i.e., {square root over ((yi2+yq2))}. Next, the output decision result estimating circuit 10 selects at most four signals which are estimated to be ya(n) (ideal output signals y(n) which would be obtained from the output signal y(n) in the absence of distortion caused by frequency-selective fading etc.) as signals P1 to P4, from among some signals y(m) preceding an nth symbol period SP. Next, the phase angle calculating circuit 11 obtains phase angles E1 to E4 between the output signal y(n) and the signals P1 to P4. These operations are performed with nth, (n+1)th, and (n+2)th output signals, yn(n), y(n+1), y(n+2), to obtain nth, (n+1)th, and (n+2)th phase angles E1 to E4, for example. The phase error selecting circuit 12 selects one phase angle with the same value from among the three sets of phase angles E1 to E4 and defines it as the actual rotation angle of the symbol S. The phase error selecting circuit 12 obtains and outputs the detected result R1 about the carrier frequency error on the basis of the rotation angle of the symbol S.
In this way, the carrier frequency error detecting circuit 5 estimates the output decision result ya(n) on the basis of the power of the output signal y(n).
In the power calculating circuit 9 of FIG. 17, if the distortion error e(n) is large, the power of the output signal y(n) also largely varies, and then the output decision result ya(n) may be erroneously estimated. Accordingly, when the distortion error e(n) is so large, the carrier frequency error detecting circuit 5 in FIG. 13 cannot normally apply feedback control to the carrier frequency error removing circuit 3, and then the carrier frequency error cannot be converged to become small.
Typically, the algorithms applied to the error detecting circuit 7 to detect the distortion error e(n) include the DD (Decision Directed) method and the CMA (Constant Modulus Algorithm) method. However, the DD method and CMA method have their respective advantages and disadvantages. That is to say:
Advantage of DD method: the distortion error e(n) becomes much smaller than in the CMA method.
Disadvantage of DD method: the distortion error e(n) cannot be normally converged if the carrier frequency error is large.
Advantage of CMA method: the distortion error e(n) is normally converged even if the carrier frequency error is large.
Disadvantage of CMA method: the distortion error e(n) does not become smaller as compared with the DD method, but it converges at a limit value L1.
The reason for the advantage of the DD method will be described. In the DD method, the difference between the output decision result ya(n) and the actual output signal y(n) is taken as the distortion error e(n). That is to say, the DD method is represented as
e(n)=y(n)xe2x88x92ya(n)xe2x80x83xe2x80x83(9)
Since the difference between the output signal y(n) and the output decision result ya(n) is directly obtained as the distortion error e(n) in the DD method, the finally obtained distortion error e(n) can be converged at a very small value.
Next, the reason for the disadvantage of the DD method will be described. At the time when the adaptive equalizer 4 in FIG. 13 starts operation, the carrier remains in the input signal x(n) without being completely removed by the carrier frequency error removing circuit 3. That is to say, a carrier frequency error may occur. When a carrier frequency error occurs, the symbol S in the output signal y(n) rotates. When the symbol S rotates, then the power of the output signal y(n) also largely varies and the output decision result ya(n) in equation (9) may be erroneously estimated. Accordingly, in the DD method, the distortion error e(n) does not normally converge if the carrier frequency error is large.
Next, the reason for the advantage of the CMA method will be described. The CMA method is expressed as
e(n)=y(n)xc2x7[|y(n)|2xe2x88x92R2]xe2x80x83xe2x80x83(10)
In equation (10), |y(n) | indicates the amplitude of the output signal y(n), and R2 indicates a fixed value. Since the distortion error e(n) is obtained with the amplitude of the output signal y(n) in the CMA method expressed by equation (10), the distortion error e(n) normally converges even if the carrier frequency error is large.
The fixed value R2 is disclosed in Godrad D.N., xe2x80x9cSelf-Recovering Equalization and Carrier Tracking in Two Dimensional Data Communication Systems,xe2x80x9d IEEE Transactions on Communications, Vol.COM-28, No.11, pp.1867-1875, November 1980 (cited reference 3). According to the cited reference 3,
R2=E[|ya(n)|4]/E[|ya(n)|2]xe2x80x83xe2x80x83(11)
In equation (11), E[|ya(n)|4] indicates an average of the fourth power of the amplitude of the output decision result ya(n), and E[|ya(n)|2] indicates an average of the square of the amplitude of the output decision result ya(n).
Next, the reason for the disadvantage of the CMA method will be described. According to equation (10), the distortion error e(n) is obtained on the basis of the difference between |y(n)|2 and the fixed value R2. In the CMA method, as compared with the DD method in which the difference between the output signal y(n) and the output decision result ya(n) is directly taken as the distortion error e(n), the distortion error e(n) does not become so smaller but converge at the limit value L1.
The cited reference 1 discloses a method which takes advantages of both of CMA method and DD method. According to the cited reference 1, first, the adaptive equalizer 4 is operated by the CMA method to remove distortion due to frequency-selective fading to a certain extent. When the distortion error e(n) has become small to a certain extent, then the carrier frequency error detecting circuit 5 can normally apply feedback control to the carrier frequency error removing circuit 3. Then, after an elapse of a predetermined time period, the carrier frequency error removing circuit 3 and the carrier frequency error detecting circuit 5 are caused to operate. Thus the carrier frequency error converges to be smaller with the passage of time. However, with the CMA method, the distortion error e(n) does not become so small, but converge at the limit value L1. Accordingly, after an elapse of a predetermined certain time period again, the adaptive equalizer 4 is caused to operate with the DD method replacing the CMA method. This allows the distortion error e(n) to become very small, thus providing an ideal output signal y(n).
However, if the time period from when the carrier has been removed completely to when the algorithm is switched to the DD method is long, that is, if the predetermined certain time period is longer than an actually required time, it uselessly takes a longer time to obtain the ideal output signal y(n). Further, if the CMA method is switched to the DD method while the distortion error e(n) is too large to allow the carrier frequency error detecting circuit 5 to normally apply feedback control to the carrier frequency error removing circuit 3, that is to say, if the predetermined certain time period is shorter than an actually required time period, then the carrier frequency error detecting circuit 5 cannot apply normal feedback control to the carrier frequency error removing circuit 3 and then the carrier frequency error does not converge to become small. This raises the problem that the ideal output signal y(n) cannot be obtained.
According to a first aspect of the present invention, an adaptive equalizer device comprises: an adaptive equalizer receiving a signal indicating a symbol, for filtering the signal indicating the symbol to adaptively remove intersymbol interference by selectively using one of a plurality of preset algorithms; and a rotation detecting circuit receiving an output signal from the adaptive equalizer, for detecting rotation of the symbol of the output signal; wherein the adaptive equalizer receives the result detected by the rotation detecting circuit to select one of the plurality of algorithms in accordance with the detected result.
Preferably, according to a second aspect of the present invention, in the adaptive equalizer device, the rotation detecting circuit comprises a symbol protrusion detecting portion receiving the output signal, for detecting whether the symbol of the output signal exceeds a predetermined reference value, and a rotation period detecting portion receiving the result detected by the symbol protrusion detecting portion, for detecting whether the detected result from the symbol protrusion detecting portion indicates that the symbol exceeds the reference value within a given period and outputting the result as the detected result of the rotation detecting circuit.
Preferably, according to a third aspect of the present invention, in the adaptive equalizer device, the symbol is composed of a plurality of dots, each of the plurality of dots being represented by components on coordinates with a plurality of axes, and wherein the symbol protrusion detecting portion comprises a plurality of component protrusion detectors receiving the output signal, for detecting whether the components of the output signal exceed the reference value for respective axes on the coordinates with the plurality of axes, and a logic portion receiving results detected by the plurality of component protrusion detectors, for outputting OR of the detected results as the detected result of the symbol protrusion detecting portion, and the rotation period detecting portion comprises a counting portion for outputting a result of comparison in magnitude between a number obtained by counting particular ones in the detected result from the symbol protrusion detecting portion in the given period and a given value as the detected result of the rotation detecting circuit.
A fourth aspect of the present invention is directed to a method for controlling an adaptive equalizer which receives a signal indicating a symbol and filters the signal indicating the symbol to adaptively remove intersymbol interference by selectively using one of a plurality of preset algorithms, wherein an output signal from the adaptive equalizer is received, and rotation of the symbol of the output signal is detected, and one of the plurality of algorithms is selected in accordance with the detected result.
Preferably, according to a fifth aspect of the present invention, in the adaptive equalizer controlling method, the detected result indicates whether the symbol exceeds a predetermined reference value within a given period.
Preferably, according to a sixth aspect of the present invention, in the adaptive equalizer controlling method, the symbol is composed of a plurality of dots, each of the plurality of dots being represented by components on coordinates with a plurality of axes, and the detected result is obtained by comparing the components on a particular one of the axes of the coordinates and the reference value.
According to the first aspect of the present invention, one of a plurality of algorithms is selected in accordance with rotation of the symbol. Accordingly, an ideal output signal can be obtained certainly and in a shorter time. Further, for example, as compared with the technique disclosed in Japanese Patent Laying-Open No.5-244040, it is possible to switch from the CMA method to the DD method by more appropriate timing.
According to the second aspect of the present invention, the symbol protrusion detecting portion and the rotation period detecting portion detect whether the symbol has protruded within a given period. This enables determination as to whether the carrier frequency error is small to such an extent that the distortion error can be normally converged by the DD method, for example.
According to the third aspect of the present invention, it is possible to know whether the symbol has got beyond the reference value by comparing components on the I axis and the Q axis and a reference value, for example. On the basis of this idea, the symbol protrusion detecting portion can be realized with a simple structure by using the component protrusion detectors and the logic portion, and the rotation period detecting portion can be realized with a simple structure by using the counting portion.
According to the fourth aspect of the present invention, the adaptive equalizer is controlled in accordance with rotation of the symbol. This allows an ideal output signal to be obtained certainly and in a shorter time. Further, as compared with the technique disclosed in Japanese Patent Laying-Open No.5-244040, for example, it is possible to switch from the CMA method to the DD method by more appropriate timing.
According to the fifth aspect of the present invention, by detecting whether the symbol has protruded beyond the reference value within a given period, it is possible to determine that the carrier frequency error is small to such an extent that the distortion error can normally converge by the DD method, for example.
According to the sixth aspect of the present invention, whether the symbol has protruded beyond the reference value can be detected by comparing components on a particular axis and the reference value, and therefore a simpler algorithm can be used to obtain the detected result from the output signal of the adaptive equalizer.
The present invention has been made to solve the above-described problems. An object of the present invention is to provide an adaptive equalizer device and a method for controlling an adaptive equalizer which can obtain an ideal output signal certainly and in a shorter time.